1. Field of the Invention
The present invention relates to wet flue gas desulfurization scrubbers generally and more particularly to the design of such scrubbers for even flue gas flow therethrough.
2. Description of the Related Art
Most wet scrubbers are designed as a spray tower. The tower is designed so that, at maximum load, the average superficial gas velocity does not exceed the design gas velocity. For most spray towers, the average gas velocity varies from about 8 to 13 ft/sec. (2.4 to 4 m/sec) based upon scrubber outlet conditions. A typical design velocity for a limestone wet scrubber is about 10 ft/sec (3.1 m/sec).
The flue gas enters the absorber from a side inlet having an awning in some cases. This tends to cause gas flow nonuniformity in the tower. This nonuniformity reduces overall SO.sub.2 removal performance and aggravates mist eliminator carryover. The absorber design incorporates a sieve or perforated plate tray which somewhat reduces the flue gas flow maldistribution. The pressure drop across the tray is usually between 1 to 3 in. Hg (0.2 to 0.7 kPa). Towers with multiple trays have also been built. The design of the tower is influenced by the reagent (lime or limestone, for example), the desired SO.sub.2 removal level, the tradeoff between fan power and recirculation lime slurry pump power, and several other factors.
Spray nozzles are used in wet scrubbers to control the mixing of lime slurry with the flue gas. The operating pressures typically vary between about 5 and 20 psi (34 and 138 kPa). Spray nozzles without internal obstructions are favored to minimize plugging by trapped debris. Although plugging could be minimized by using a minimum number of large spray nozzles, flow maldistribution would most likely occur. Therefore, several smaller nozzles are usually preferred. Ceramic nozzles are commonly used.
The large lime slurry tank at the bottom of the spray tower is called the reaction tank or the recirculation tank. The volume of this tank permits several chemical and physical processes to approach completion.
Gas liquid contacting is essential in the described towers to achieving high efficiency pollutant removal and to improve reagent utilization. Gas distribution suffers as the inlet gas velocity in particular and the absorber gas velocity in general increase. Field data at a nominal average gas velocity 15.9 fps showed that the gas flow in the rear of the absorber under the tray was 16% above the average, while the gas flow in the front was 23% below the average. The corresponding values above the tray were 13%, 92%, and 96% of the average flow. Meanwhile, at a nominal average gas velocity of 10 fps the gas distribution under and above the tray indicated that the gas flow is more evenly distributed.
As the gas distribution in the absorber tower is distorted, the gas/liquid disproportionation becomes significant and possibly damaging to the process efficiency as the gas velocity exceeds 11.5 fps and approaches 20 fps. Two factors affect the gas distribution:
1. The momentum of the entering gas forces the gas towards the rear of the absorber allowing the liquid to flow unchallenged at the front of the vessel. PA1 2. As the gas and liquid segregate and the liquid flows towards the front of the absorber, the resistance to the gas increases thus forcing the gas to enter the vessel from the sides of the inlet between the side shields of the awning to the sides and the liquid curtain falling in the middle of the inlet. This distortion of flow aggravates the flow distribution problem. PA1 1. Reduce the gas inlet velocity without increasing the absorber height. PA1 2. Introduce the gas in a manner that promotes even gas distribution and adequate humidification. PA1 3. Reduce the resistance of the liquid falling from the absorption section of the absorber to the entering gas. PA1 4. Distribute the gas and liquid evenly across the inlet resulting in a lower inlet pressure drop, thorough humidification of the entering gas, provide maximum gas liquid contact, and improve the reagent utilization. PA1 To accomplish this the scrubber inlet is designed so as to introduce the gas through multiple inlet ports located in the scrubber tower without a substantial increase in the absorber tower height. This eliminates the need for an inlet awning and utilizes the absorber walls to perform the traditional functions of the awning. The new inlet ports are located in the inclined transition surface between the flared tank and the absorption section of the absorber. The incoming flue inlet connects to a plenum that surrounds the lower portion of the absorption tower section. Openings or ports are located radially around the circumference of the plenum which is located around the inclined transition between the reagent tank and the absorption section of the tower. The size, number and location of the openings varies from the front of the absorber (the direction where the gas flue comes) to the rear. The front port openings are made smaller than the back to force the gas through the plenum to the back openings of the plenum. Splitters may be located at proper locations in the flue and are used to help redistribution of the gas.
Awnings at the side flue gas inlet, while protecting the inlet from wet/dry interface growths and wetness, contribute to the problem by providing resistance to the gas at the center of the inlet. The thick liquid layer falling off the awning forces the gas to enter the absorber from the sides. The gas follows the absorber walls to the rear end of the absorber and loses its momentum as it turns upward at the rear of the absorber. As the gas flows towards the rear of the absorber, it forces the liquid toward the front increasing the density of the awning liquid curtain which in turn force the gas to the sides. In addition to the damaging potential of gas maldistribution, maintaining the same inlet flow area required for a 10 fps absorber, the high gas velocity in the inlet results in a high inlet pressure drop and high gas momentum which contributes to the gas maldistribution problem.
Reducing the inlet gas velocity to the acceptable 3000 fpm will result in either a very wide inlet that complicates the mechanical design of the inlet, or requires increases in the inlet height which defeats the purpose of high velocity absorbers namely to reduce the size of the vessel.
Thus the prior art fails to provide an absorber tower which would meet the following requirements: